New York has declared a public health emergency following a measles outbreak.
The emergency warning covers certain postcodes in Brooklyn. All residents in the affected areas have been told to get vaccinated or face a fine.
There have been 285 cases in the area since 30 September. Of those infected, 246 were children, New York health commissioner Oxiris Barbot said.
It is the largest measles outbreak in the city since 1991.
Measles is a highly contagious viral disease that can result in serious health complications, such as pneumonia and swelling of the brain.
In a press conference, Dr Barbot said that of the 285 reported cases, 21 people have been hospitalised and five had been admitted to an intensive care unit.
No deaths have been reported.
Under the order, any person living in the affected areas who has not been vaccinated must be immunised within 48 hours.
Parents of children older than six months must get them vaccinated or show proof of immunisation.
Mayor Bill de Blasio told reporters: "I want everyone to understand how serious this is and how quickly it spreads. The bottom line is to recognise that this is something that has now become even more urgent.
In March. a county in New York state declared a state of emergency following a measles outbreak.
Rockland County banned unvaccinated children from public spaces after 153 cases were confirmed.